Sorry for the late response. All stats and graphs are based upon weekly Tatoeba database dumps. The last dump was performed at 9:00 (GMT) on December,1st, so "December" columns on the Top 10 Most Active Languages Graph show only those few sentences added till that time.

If 9 hours of the first day of December represent the whole month of December on the graphs it does make sense. It's like everyone almost stopped contributing in December comparing to the previous months.